Treatment of finely divided magnetic material



April 8, 1947. w. c. ELLIS EI'AL 2,418,467

TREATMENT OF FINELY DIVIDED memmc MATERIAL Filed Sept. 24, 1943 FIG. I

A0 A7 50 MEGACYCLES I w c ELL/S 'ZZ A a. SOUDE'N Patented Apr. 8,1947

ICE

* TREATMENT or FINELY DIVIDED MAGNETIC MATERIAL William 0. Ellis,Maplewood, and Alexander G.

Souden, Summit, N. J., assignors to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York H ApplicationSeptember 24, 1943, Serial No. 503,718

13 Claims. (01. 23-290) This invention relates to magnetic bodiescomprising finely divided magnetic material, and

more particularly to the treatment of finely di-.

vided magnetic material to improve its properties for use in suchmagnetic bodies.

Finely divided magnetic material to the treatment of which the presentinvention pertains may be employed in magnetic bodies of the compressedtype, i. e., those formed by compressing a finely divided magneticmaterial, with or without an added electrical insulating material, underhigh pressures to form a body of dense structure. Said finely dividedmagnetic material may also be employed in magnetic bodies made ofloosely assembled particles of magnetic material with or without addedelectrical insulating material therebetween. The magnetic bodies formedof the finely divided magnetic material treated according to the presentinvention may be employed to particularly great advantage in highfrequency electrical circuits operating at frequencies of at least about25 megacycles; they may also be employed, although to less advantage, incircuits operating at lower frequencies such as telephone, broadcastradio and other circuits.

Because of the characteristics of the magnetic material treatedaccording to the present invention, it may be employed to particularadvantage in compressed magnetic cores for inductance coils in highfrequency circuits. For the purpose of illustration but not oflimitation the magnetic material treated according to this inventionwill be largely discussed hereinafter in connection with such cores.

It has been proposed to employ an oxide of the ferro-ferric oxide typein magnetic bodies. However, it has been found that such an oxide, evenif synthetically produced and hence presumably produced under controlledconditions which will provide uniformity of composition and substantialfreedom from impurities, does not provide as good results as are desiredwhen the oxide is employed in magnetic cores for inductance coils,particularly those cores employed in high frequency circuits. In coresemployed in such circuits losses in the cores must be low; this andother magnetic characteristics of the cores should be highly uniformamong all cores of the same type which are manufactured; and thecharacteristics of the cores should not change substantially duringoperation. It has been found that cores formed of the commerciallyavailable finely divided ferroso-ferric oxides often have losscharacteristics which are excessively high; in other cases cores formedof such oxides which have initially satisfactory low losscharacteristics develop excessive losses and deleterious changes inother magnetic characteristics during operation, particularly when thecores are employed under conditions at which they reach relatively hightemperatures and even more particularly when employed under conditionsof varying temperatures; in other cases cores formed of ferroso-ferricoxides have losses: which are initiallyexcessively high and becomeprogressively worse during operation.

In accordance with the present invention, these disadvantages ofmagnetic bodies comprising finely divided ferroso-ferric oxide areminimized, if not entirely eliminated, by the employment of aferroso-ferric oxide which has been treated according to the presentinvention. According to the present invention, finely dividedferroso-ferric oxide of a particular composition indicated hereafter isheated in an inert atmosphere to an elevated temperature which isadvantageously in a critical temperature range. Such heat treatment ofthe oxide reduces excessive losses which might be present in coresformed of the-untreated oxide, and stabilizes the magneticcharacteristics of the cores so that their losses do not increase andtheir other magnetic characteristics do not deleteriously change duringoperation even at elevated or varying temperatures.

The exact nature and advantages of the present invention will be betterunderstood from the following detailed description in connection wit theaccompanying drawings in which:

Fig. 1 represents a curve illustrating the reduction in losses which maybe obtained in magnetic cores formed of finely divided ferroso-ferricoxide treated according to the present invention; and

Fig. 2 illustrates one form of core formed of finely dividedferroso-ferric oxide treated accord ing to the present invention.

Ferroso-ferric oxide theoretically has a composition corresponding tothe chemical formula F8304 and is usually .considered to be a ferrousferrite of the formula FeQFezOa. In the process of the presentinvention, however, the ferroso-ferric oxide heat treated is one inwhich the F620: component is in substantial molecular excess withrespect to the FeO compo-.

nent; that is, the molal ratio of FezOa/FeO is substantially greaterthan one. As an example, marked advantages are provided if, in aferrosoferric oxide heat treated according to the present invention, themolal ratio FezOa/FeO lies between about 1.25 and 1.75; exceptionallygood results have been obtained with finely divided ferrosoferric oxideshaving a molal ratio of between about 1.40 and 1.55. Moreover, theferroso-ferric oxide is substantially free of impurities; itadvantageously containsless than about 0.03 per cent by weight ofmetallic iron and only comparable amounts of other impurities when it isto be employed in ,cores used in high frequency circuits. Higherproportions oi impurities might impair magnetic characteristics at highfrequencies of about 25 megacycles and up. The size 01' the particles ofthe finely divided Ierroso-ferric oxide does not appear to affect theresults obtained by heat treatment; the only limitations as to particlesize appear to be those imposed by the requirements of the use to whichthe oxide is put; Satisfactory compressed cores may be made from afinely divided oxide which before incorporation into the cores hasparticles of such a size that all pass through a No. 40 screen; ofcourse, oxides having all particles of a smaller size may be employed.However, particularly advantageous results are obtained in reducinglosses of magnetic bodies by heat treatment of the finely divided oxidewhen the oxide before heat treatment according to the present inventionhas particles of such size and shape that the tap density of the oxideis between about .8 to about .85. The tap density is determined byfilling a 100 cubic centimeter graduated cylinder with the finelydivided oxide, closing the end with a stopper, and tapping the graduatecylinder one hundred times onto a wooden surface from a, height of 8inches; the tap density is the weight of the oxide in grams divided bythe volume of the oxide in cubic centimeters. v

In accordance with the present invention, such an oxide isadvantageously heated in an inert atmosphere to a temperature betweenabout 980 C. and about 1020" 0.; preferably it is heated to atemperature of about 1000" C. The time of heating is such as to impartthe desired characteristics to the oxide.

The heating may be accomplished in various manners. Advantageously, thefinely divided ferroso-ferric'oxide is heated while it is in a mass ofloose particles before being incorporated into the magnetic body. At thetemperatures and under the conditions employed in the process of theinvention no, or very little, sintering together of the oxide particlesoccurs. If desired, a suitable finely divided electrical insulatingmaterial capable of resisting the heat treatment temperatures, such asan inorganic ceramic type insulating material comprising talc or kaolin,may be mixed with the mass of loose oxide particles befcre they aresubjected to heat; the'presence of the insulating material reduces thepossibility of sintering of the oxide particles. The improvement inproperties of the ferroso-ferric oxide may also be obtained if thefinely divided oxide is heated after it has been formed into a magneticbody either of the compressed type or the loose particle type. However,in general, it is advantageous to heat the particles when they are inloose form with or without an added insulating material since thisreduces the possibility of sintering together of the oxide particles,which tends to cause a loss of desirable magnetic characteristics.

The inert atmosphere may be provided in various manners. Thus theferroso-ferric oxide being treated, either in loose form or otherwise,may be heated in a vacuum or in an inert gas of which nitrogen is anexample. However, for maximum ease in carrying out the process andfreedom from complexity of the heating apparatus required, it is moredesirable to heat the oxide while it is contained in a sealed potcontaining substantially no air. The pot should be formed of a materialwhich does not tend to combine with the oxide a pot formed of a,refractory nickel-ironhromium alloy metal, with or without an aluminumoxide lining, has been found satisfactory.

The high temperatures employed may be obtained in different ways. Forexample, the oxide may be heated in an electrical furnace or in agasfired Iurnace,.care being taken in each case to insure that the oxideis heated under an inert atmosphere.

The time of heating is not critical. Improvements in the properties ofthe ferroso-ferric oxide have been obtained when the oxide was heatedfor as short a time as one-half hour and as long a time as five hours;heating times of longer than five hours may be employed. From thecommer-- cial standpoint, of course, shorter heating times aredesirable. It has, been found that highly advantageous results areobtained when the finely divided oxide is heated from one to one andonehalf hours.

The ferroso-ferric oxide may be cooled from the heat treatmenttemperatures at any suitable rate. In general. it is merely necessary tocool the oxide without any particular precautions to change or controlthe rate of cooling, provided the oxide is maintained in an inertatmosphere during at least the time it is at a substantially elevatedtemperature.

The temperature at which best results are obtained when the oxide isheated isquite critical, as is apparent from the curve of Fig. 1. Inobtaining the data for this curve, each of several masses of a looseferroso-ferric oxide powder of the kind described above was respectivelyheated in a nitrogen atmosphere for one and one-half hours to adifferent temperature which tell between temperature 700" C. and 11000., one of the masses of powder being heated to 1000" C. From each ofthese masses of powder, without added insulation, a toroidal core of anidentical predetermined size was pressed at a pressure of 50 tons persquare inch. The quantity AQ was then determined for each of said cores.

The quantity Q for an inductance coil is given where F is the frequencyof the current, L is the inductance of the coil and R is its resistance.The quantity AQ for a core is equal to the difference between the Q of acoil alone and the Q of the same coil with the magnetic core therein; itthus indicates the magnitude of the losses of the core. A core with apositive AQ Value or a low negative AQ value has low loss"characteristics. The quantity AQ for each of the cores tested inobtaining the values for the curve'of Fig. 1 was determined by means ofa Q-meter of the type described on page 15 of the May-1942 .issue of themagazine FM Radi0Electronic Engineer ing and Design, published by the FMCompany, New York city, and by a process generally similar to thatdescribed therein. The Q-meter was provided with an inductance coil intowhich one of the cores could be inserted. The effective Q of the coilwithout the core was determined as described on said page of saidpublication at a frequency of 50 megacycles; the effective Q of the coilwith the core inserted therein was determined at the same frequency inthe same manner; the quantity AQ corresponding to the difference inthese Q values was an indication of the losses introduced by the core.

As is apparent from Fig. 1, heating of the, ferroso-ferric oxide testedto a temperature on the order of 1000 0. reduced the core lossessubstantialLv and to a much greater extent than did heating of the oxideto temperatures below and above this temperature. The curve of Fig. 1thus shows that the heating temperature range indicated above, fromabout 980 C. to about 1020" C. 3

is quite critical in providing maximum improvement in properties.

The above curve is purely illustrative in indicating the eilfects oftemperature of heating of a particular oxide on core losses of aspecific type of core; different curves may be obtained when diflerentoxides are employed or when cores of different sizes and shapes aretested. For example, cores having a AQ value of over +5 at a frequencyof 50 megacycles may be made of oxide treated according to the presentinvention.

The finely divided ferroso-ferric oxide after treatment according to thepresent invention possesses permeability and other magneticcharacteristics which render the material highly advan tageous for usein magnetic bodies, particularly bodies employed in high frequencycircuits operating at frequencies between about 25 and about 200megacycles. The characteristics of the heat treated oxide are such thatwhen formed into magnetic bodies such as cores, the bodies have low losscharacteristics, and the loss characteristics and other magneticproperties of the bodies are extremely stable so that they do not changesubstantially even if the bodies are employed at high temperatures or atfluctuating temperatures.

More specifically, heat treatment, according to the present invention,of finely divided ferrosoferric oxide of the kind the heat treatment ofwhich is contemplated by the present invention substantially reduces thelosses of magnetic bod ies formed of the oxide if the oxide originallycauses high losses, and does not appreciably harm the stability of theloss characteristics and other magnetic properties of the bodies if theuntreated oxide produces bodies possessing such stability; in othercases in which the oxide before heat treatment does not cause highlosses, heat treatment according to the present invention does notincrease such losses but greatly increases the stability of the losscharacteristics and the magnetic properties of the bodies even if theyare employed at high or fluctuating temperatures; in still other casesin which the oxide before heat treatment produces magnetic bodies havingboth high loss characteristics and poor stability of losscharacteristics and other magnetic properties, heat treatments of theoxide according to the present invention reduces the losses of themagnetic bodies and greatly improves the stability of the losscharacteristics and other magnetic properties even at high orfluctuating temperatures.

The reasons are not definitely known for the improvement of the losscharacteristics and other magnetic properties, and the improvedstability thereof, of magnetic bodies comprising the finely dividedferroso-ferric oxide heat treated according to the invention.Examination of the oxide before and after heat treatment by means ofX-rays has not indicated any substantial recrystallization; the sizes ofthe particles of the finely divided oxide after heat treatment aresubstantially the same as the sizes of the particles before treatment.

As has been indicated above, in general, substantially no sintering ofthe particles of the oxide occurs when the oxide is heated in loose formwith or without an added insulating material. Grinding of the heattreated oxide is 6 unnecessary; at most a sieving of the heat treatedoxide may be desirable.

Ferroso-ferric oxide which has been heattreated according to the presentinvention while in a loose mass may be formed into compressed cores withor without separate insulation material. Figure 2 illustrates one formof core embodylng oxide heat-treated according to the invention.

In general it is desirable to employ separate insulation material tocontrol or adjust the permeability characteristics of the resultingcores. Various types of organic and inorganic insulating materials maybe employed, such as are known to the art. As examples of organicinsulating materials may be mentioned thermosetting organic resins suchas phenol-formaldehyde condensation products. As examples of suitableinorganic insulating materials may be mentioned the ceramic insulatingmaterials disclosed in Patent 1,943,115, issued January 9, 1934, to W.C. Ellis. The compressed cores may be formed by procedures known tothosein the art. For example, the heat-treated finely divided ox,- ideparticles may be mixed with and coated with a suitable ceramicinsulation in liquid form, dried, and then pressed into cores with orwithout the addition of another insulating material of the ceramic ororganic type. Or the heat-treated oxide particles may be mixed withinorganic or organic insulation and pressed into core shape.

As has been indicated above, ferroso-ferric oxide which has beenheat-treated while in a loose mass according to the present inventionmay be employed in cores of the loose powder type'if desired. As wasalso indicated above, the finely divided ferroso-ferric oxide may beformed into compressed or loose type cores which after formation areheat-treated according to the present invention. If an insulatingmaterial is used in such a core, it is necessary to employ one: whichresists the high temperatures employed, such as a ceramic insulation.

The following is an example of a preferred process of the presentinvention:

Finely divided ferroso-ferric oxide, sold com mercially as a syntheticchemi ally pure grade, having a mole] FezOs/FeO ratio lgf 1.4 andcontaining less than 0.03 per cent metallic iron and only traces ofother impurities, all the particles of which oxide passed through a No.40 screen, and which oxide had a tap density of .811, was placed inloose form in a nickel-iron-chromium alloy metal pot lined with aluminumoxide. The pot was filled with said oxide and was sealed to excludesubstantially all air. The sealed pot was then placed in a previouslyheated furnace and the finely divided oxide was heated at a temperatureof about 1000 C., as determined by a thermo-couple, for about one andone-half hours. The pot was removed from the furnace and allowed to coolto room temperature before the seal was broken. Any red oxide found onthe surface of the heat treated oxide was removed and discarded. Theremaining black oxide was passed through a No. 40 screen. Cores of theform shown in Fig. 2 and pressedfrom the heat-treated powder at apressure of 50 tons per square inch at frequencies on the order of 50megacycles had loss characteristics only about one-fifth as great asthose of identical cores pressed from the identical powder which was notheat treated, the loss characteristics being determined as describedabove in connection with Fig. 1.

While the invention has been primarily disin the treatment offerroso-ferric oxide powder for other magnetic purposes. Variousmodifications' may be made in the methods of heating, and in otherieatures of'the process described above. It is intended that the patentshall cover by suitable expression of the appended claims whateverfeatures of patentable novelty reside in the invention.

What is claimed is:

1. A methodof improving the properties of a ferroso-terric oxiderendering it suitable for use in a magnetic body comprising heating afinely divided ferroso-ferric oxide comprising a substantialmolecularexcess oi F6203 with respect to FeO and being substantiallyfree of impurities, to a temperature in'excess of 700 C. in an inertatmosphere for a period of at least about onehalf hour.

2. A method of improving the properties of a ferroso-ferric oxiderendering it suitable for use in a magnetic body comprising heating afinely divided ferroso-ferric oxide comprising a substantial molecularexcess .of FezOa with respect to FeO and being substantially free ofimpurities, to a temperature between about 980 C. and about 1020 C. inan inert atmosphere for at least about one-half hour.

3. A method of improvingthe properties .of a ferroso-ferric oxiderendering it suitable for use in a magnetic body to be employed in acircuit operating at a. frequency of at least about 25 megacyclescomprising heating a finely divided ierroso-ferric oxide comprising amolecular proportion of F6203 between about 1.25 and 1.75 times as greatas the molecular proportion of FeO and containing less than about 0.03per cent by weight of metallic iron and substantially no otherimpurities, to a temperature between about 980 C. and about 1020 C. inan inert atmosphere for at least about one-half hour.

4. A method of improving the properties of a ferroso-ferric oxiderendering it suitable for use in a magnetic body comprising heating aloose mass of a finely divided ferroso-ferric oxide comprising asubstantial molecular excess of F6203 with respect to FeO and beingsubstantially free of impurities, to a temperature between about 980 C.and about 1020 C. in an inert atmosterroso-ferric oxide rendering itsuitable for use in a magnetic body comprising heating a loose mass of afinely divided ierroso-ferric oxide comprising a substantial molecularexcess of FezOs with respect to FeO and being substantially free ofimpurities, to .a temperature between about 980 C. and about 1020 C. inan atmosphere of an inert gas for from about one-half hour to about fivehours.

' 8. A method of improving the properties of a Ierroso-ferric oxiderendering it suitable for use.

in a magnetic body comprising heating a loose mass of a finely dividedferroso-ferric oxide phere for a period of at least about one-half hour.

5. A method of improving the properties of a ferroso-ferric oxiderendering it suitable for use 1 in a magnetic body comprising heating aloose mass of a, finely divided ferroso-ferric oxide comprising asubstantial molecular excess of FezOa with respect to FeO and beingsubstantially free of impurities, to a temperature between about 980 C.and about 1020 C. in an atmosphere of an inert gas for a period of atleast about onecomprising a substantial molecular excess of FezO; withrespect to FeO and being substantially freeof impurities, to atemperature between about 980 C. and about 1020 C. in a sealed potcontaining a substantially inert atmosphere for from about one-half hourto about five hours.

' 9. A method of improving the properties of a ierroso-ferric oxiderendering it suitable for use in magnetic bodies comprising heating aloose mass of a finely divided ferroso-ferric oxide comprising amolecular proportion of F8203 between about 1.40 to 1.55 times as greatas the molecular proportion of FeO and containing less than about 0.03per cent by weight of metallic iron and substantially no otherimpurities, and having a tap density between about .8 and .85 to atemperature between about 980 C. and about 1020 C. in an inertatmosphere for at least about one-half hour.

10. The method of improving the properties of magnetic bodies comprisingheating a pressed .magnetic body comprising a finely divided ferroso-11. The method of improving the properties of magnetic cores comprisingheating a pressed magnetic core comprising a finely dividedferrosoferric oxide comprising a molecular proportion of F6201 betweenabout 1.40 to 1.55 times as great as the molecular proportion of FeO andcontaining less than about 0.03 per cent by weight of metallic iron andsubstantially no other impurities, and having a tap density betweenabout .8 and about .85 to a temperature between about 980 C. and about1020 C. in an inert atmosphere for at least about five hours.

12. Process of claim 11 in which said inert atmosphere is an atmosphereof an inert gas surrounding said core.

13. Process of claim 11 in which said core is heated in a substantiallyevacuated container.

- WILLIAM C. ELLIS.

ALEXANDER G. SOUDEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

